Semiconducting polymers have been rapidly developed over the past decades. Solar cell and transistor devices have been designed that are based upon solution processable organic semiconducting polymers. Preparation of these devices is commercially attractive from the expectation that processing these semiconductors by printing methods is potentially much less expensive than the equivalent devices constructed from conventional inorganic materials. If material design is successful organic electronics would be an attractive alternative to the established technologies. The two major barriers to introduction of organic-based devices has been their relative performance and the stability of organic-based devices over time, which are inferior to devices based on inorganic semiconductor materials such as silicon. Improvements in performance metrics, such as power conversion efficiency in solar cells and charge mobility in transistors, could rapidly result in new and larger markets for any of these materials that have adequate stability at ambient conditions.
Poly(3-hexylthiophene) (P3HT) has been the most attractive organic material for transistor and photovoltaic based devices since 2006, and has been extensively developed. However, P3HT has a maximum field-effect charge mobility of around 0.1 cm2V−1s−1. An approach taken toward increasing the charge carrier mobility in organic polymers has focused on fused aromatic rings to assure planarity of the aromatic units, which effectively extends the conjugation length and allows greater delocalization of injected charge carriers along a polymer backbone. In addition to the increase of conjugation length, fusion of rings promotes pi-pi stacking and other favorable intermolecular interactions between the large area coplanar aromatic segments of adjacent polymer chains to allow relatively efficient electrical transfer between chains. However, homopolymers of fused heterocycles often tend to be unstable, and the HOMO-LUMO levels of these homopolymers are generally not aligned with those of the fullerenes, limiting their use in bulk heterojunction solar cells
The stability shortcomings of fused ring homopolymers have been overcome by copolymerization. For example cyclopentadithiophene (CPDT), shown below, has been incorporated in copolymers that achieve charge mobilities of more than 1 cm2V−1s−1 in a transistor, and greater than 5% power conversion efficiency (PCE) in solar cells. Copolymers based on dithienosilole (DTS), shown below, display charge carrier mobilities approaching 1 cm2V−1s−1, and have displayed PCE's that exceed 6%.

A donor-acceptor D-A approach to copolymers has allowed the tuning of frontier orbital energy levels of copolymers, allowing the modification of absorption by these materials. Tuning of the ultraviolet, visible, and near-infrared absorption bands of conjugated copolymers has been achieved by the alternation of electron-rich (donor, D) and electron-poor (acceptor, A) segments. The D-A copolymer approach has been used to tune the copolymer structure to achieve favorable optical and electronic properties for application such as field-effect transistors, light emitting diodes, and photovoltaics. An additional advantage of using the D-A copolymer approach has been greater stability to ambient atmosphere conditions due to a decrease in the energy level of both occupied and unoccupied molecular orbitals.
As indicated above, the inclusion of the silicon atom for a carbon atom in the fused ring unit allows relatively high charge carrier mobilities, which has been attributed to the interaction of the σ* orbital of the Si with the π* orbitals of the conjugated carbon system and to the changes in steric considerations due to the increased bond length of C—Si bonds over C—C bonds in otherwise equivalent polymers. This difference does not only change the structure of the individual copolymer chains, but also effects the interaction of adjacent chains, for example the ability to stack the flat aromatic groups of nearest neighbor chains, allowing closer, better aligned interactions are facilitated on the intermolecular level. To this end, improvements by the inclusion of Ge for C or Si is of interest; however, there are few examples of fused ring Ge molecules or polymers containing these molecules.